FIELD OF THE INVENTION
The invention relates to a device for introducing a liquid reducing agent into an exhaust gas purification system. It is used particularly, in an internal combustion engine, in the exhaust gas purification system equipped with a regulated or controlled diesel catalyst ("GDK"). It may also be used in an exhaust gas purification system for stationary diesel engines, for example, with a mechanical power of up to a 1000 kW.
Above all other methods, the technique using regulated or controlled diesel catalyst GDK has proved to be advantageous in order to reduce the pollutants, in particular, nitrogen oxides, contained in the exhaust gas of an internal combustion engine operated with air excess, such as, for example, in diesel and lean-burn engines. In the technique, which is based essentially on the method of selective catalytic reduction ("SCR"), the nitrogen oxides are brought into contact with ammonia at a selective catalyst and are converted there into nitrogen and water.
Due to risks associated with using ammonia, specifically, its toxicity, and because of the troublesome smell generated by ammonia, ammonia is not generally carried in a vehicle using an internal combustion engine equipped with a GDK system. The reducing agent necessary for the catalytic conversion of the nitrogen oxides is, therefore, transported in the vehicle in liquid form as an aqueous urea solution. The ammonia is produced from the aqueous urea solution by hydrolysis in a precise quantity required at a particular moment for conversion of the nitrogen oxides. Where stationary smoke gas purification systems are concerned, for example, downstream of power stations, pure ammonia or ammonia water can be used.
German Published, Non-Prosecuted Patent Application DE 44 17 238, provides for directly leading the exhaust line of a truck diesel engine laterally up to a cylindrical inlet chamber, in which is disposed a funnel-shaped perforated plate. At the narrowest point of the funnel, an injection valve is provided, through which an aqueous urea solution is injected into the inner space of the funnel. A homogenous distribution of the urea solution in the exhaust gas over the entire cross section of the inlet chamber is achieved. A hydrolysis catalyst, a DeNOx catalyst and, if appropriate, an oxidation catalyst, follows the inlet chamber.
A solution differing from application DE 44 17 238 is disclosed in International PCT publication WO 96/36797. In order to obtain a sufficient nebulization of the liquid reducing agent to be atomized, i.e., the reducing agent urea, before introduction into the pollutant-laden exhaust gas stream, a mixing device or mixing chamber is provided there. Into the mixing chamber are introduced the liquid reducing agent and the gas, for example, air, for intimate mixing with one another, i.e., to form an emulsion. The mixing chamber is connected through a single mixing line or pipeline to an atomizer nozzle that is disposed in the exhaust gas stream. An adjustable metering valve precedes the mixing chamber. The metering valve aids in the predetermination of the reducing agent quantity necessary per unit time. The reducing agent quantity flowing through the opened metering valve per unit time is directly dependent on the differential pressure across the metering valve. In order to ensure a constant metering rate for the reducing agent in the prior art device, it is necessary to have a constant differential pressure.
The differential pressure prevailing across the metering valve in a prior art device depends both on the pressure in the reducing agent line upstream of the metering valve and on the pressure in the mixing chamber. Therefore, the differential pressure also depends on the gas pressure in the gas line leading into the mixing chamber. A pump preceding the metering valve for conveying the reducing agent generates the pressure. However, both the gas pressure and the pressure in the reducing agent line may experience fluctuations.